Coating formulations or “colors” used to coat paper or paperboard products are usually applied to at least one side of a paper base stock to improve surface smoothness and to provide uniform ink reception and other generally acceptable printing properties. Typically, such formulations include pigments, binders and other additives such as dispersants, viscosity modifiers, lubricants, and chemicals that modify foaming tendencies, microbial susceptibility, pH or physical characteristics such as; color. Porous pigments, such as alumina, are important components of such coating formulations because they contribute to surface porosity of the paper and ink drainage. The porous pigments also reduce surface irregularities, thus improving paper smoothness. In general, a higher proportion of porous pigment improves the smoothness, porosity and ink-receptivity of the coating. These characteristics are particularly desirable for certain coated paper product used in inkjet printing. A commonly observed drawback, however, is that the particulate nature of the pigment limits the amount that can effectively used in the coating formulation.
The pigment is usually added during make-down of the coating formulation as a dispersion or slurry of finely divided particles, as this mode of addition is more convenient for bulk manufacturing. The use of such dispersions is, however, limited by the amount of pigment that is actually suspended in the dispersing medium at the time that it is added to the coating formulation. Because the pigments are insoluble, the pigment particles will settle out, resulting in a reduced amount of pigment per unit volume in the dispersion and the coating color. As a consequence, it is difficult to control the total solids content in the dispersion itself and in the final coating formulation. Another disadvantage is that the particles may react with certain components of the dispersion and aggregate to form a gel or sol of high viscosity, which may cause an undesirable increase in the viscosity or otherwise affect the rheological properties of the coating color.
Inventive efforts have therefore focused on the preparation of stable dispersions of pigments to be incorporated into coating formulations in a manner that allows maximum compatibility with other components of the coating formulation. Such dispersions have been prepared using an acid-based dispersant as a means of stabilizing the pigment particles in suspension. For example, U.S. Pat. No. 3,935,023 (Derolf) discloses aqueous dispersions of alumina in a hydrochloric acid-containing solution. The dispersions so formed have an alumina content of from about 18–26% weight, based on the total weight of solids, or about 54–66% parts by weight (pbw) of Al2O3 per pbw of hydrochloric acid, and a specific surface area of pigment, as determined by the Brunauer, Emmett and Teller (BET) method of 150–250 m2/gram. The Derolf patent teaches that in order to achieve dispersions of 18–26% by weight, it is necessary to maintain an Al2O3 to HCl ratio of 54–65 pbw to 1 pbw of HCl, and that when less than 53 pbw of Al2O3 per pbw of HCl is used, the shelf life of the resulting product is drastically limited. Accordingly, the acid is a necessary component of Derolf's coating formulations. However, acids are hazardous and corrosive materials and are less effective at dispersing pigments. Moreover, it has been determined experimentally that using acids as the primary dispersion means does not produce slurries with a high level of pigment solids at an desired viscosities.
U.S. Pat. No. 5,518,660 (Wei) describes preparing low-viscosity colloidal dispersions of sub-micrometer alumina particles for use in ceramics manufacture that contain 0.1 to 5% by weight of a semicarbazide hydrochloride acidic compound as a dispersing agent. In this regard, the semicarbazide compound dissociates in water by releasing the HCl ligand, which, in turn acidizes the dispersion to avoid agglomeration and thereby maintaining a low viscosity. The Wei method has the short comings similar to Derolf.
As an alternative, coating formulation have been prepared by incorporating the solid pigment directly into the color formulation. For example, Japanese Patent Abstract No. JP 9314985A2 describes an aqueous coating formulation for inkjet or electrophotographic printing containing alumina hydrate, ethylenediamine-tetraacetic acid sodium as a chelating agent, an imidazolium compound as a cationic component and polyacrylamide. The process described by the patent abstract eliminates the step of dispersing the alumina hydrate and adding it to the coating formulation as a slurry. Direct introduction of a solid pigment during preparation of the coating formulation as reported in this abstract is undesirable, however, because additional steps are then required to homogenize the coating formulation and to maintain the desired Theological properties. These additional steps would increase manufacturing costs. In addition, the coating formulation may still be susceptible to the deterioration observed with respect to many dispersions, such as flocculation and gellation.
Accordingly, a need exists for a method of making and applying a coating formulation to paper wherein the coating has a high pigment solids content.